The present invention is directed to pressure measuring probe systems and, more particularly, to probe systems for use in highly turbulent fluid streams that measure time-averaged static or ambient pressure and turbulence intensity.
The static or ambient pressure in a fluid stream is defined as the pressure that would be sensed if an observer or instrument were to move with the local stream direction and velocity, i.e., float along with an element of the fluid. Since it is usually impractical to have a pressure sensing device float along with a local stream element, it has been common practice to use a tubular probe having orifices in its side, the probe pointing into the stream direction. The location of the orifices is such that, when the flow is steady, they sense the local static pressure. Such a technique provides a reliable measurement for the ambient pressure because the probe is designed to cause negligible distortion to the local pressure field at the location of the orifices in the side of the tubular probe.
When the flow is unsteady or turbulent, a tubular probe can no longer be aligned with the local streamline direction at all times because the flow direction is changing continuously. Turbulent velocity fluctuations then cause significant pressure variations at the orifices in the probe. Under those circumstances, the instantaneous flow has a velocity component across the probe axis a large portion of the time. When the fluid flows across or around a tube, the local pressure field is often strongly perturbed by the presence of the tube. Hence, not only is an average of the static pressure required over time, but the pressure measured at the orifices is not clearly related to the actual local static pressure which would exist in the absence of the probe. It would be preferable to design a probe so that the unwanted contributions to pressure due to the unsteady cross-flow would be negligible, as they are in a steady stream, thereby enabling direct measurement. It is very difficult to design a probe such that unwanted contributions are not registered, because the sides of the probe would have to be parallel to the local stream direction or ride along with the fluid.
A wide variety of probe shapes, as shown in FIGS. 1A-1H, have been used to measure the static or ambient pressure in both laminar and turbulent streams. FIG. 1A illustrates the Prandtl probe 100 (Prandtl et al., Applied Hydro- and Aeromechanics, McGraw Hill Book Co., Inc., 1934) which is imbedded in the stream and which is capable of making both a total head and a static pressure measurement. The probe has a hemispherical nose 102 with an orifice 104 connected to an axial duct 104' on the centerline for measurement of stagnation pressure. In order to measure the static pressure on the same streamline, the probe has a ring of orifices 106 through the side walls of the tube located about five diameters downstream of the nose, which are connected to an axial, annular duct 106'. Prandtl also made recommendations for the design of the installation so that the effects of the probe and its support on the measurements are minimized. The Prandtl probe yields results accurate to within several percent when its inclination to the flow is small, but significant error develops in the measurement of both static and total pressure if the flow angle relative to the probe axis is larger than about 10.degree.. The pressure at the static orifices is thus sensitive to the cross-flow that occurs when mean flow incidence and/or velocity fluctuations are present.
The wedge-shaped static probe 110 illustrated in FIG. 1B and described by Gettelman & Krause (Gettelman et al., "Characteristics of a Wedge with Various Holder Configurations for Static-Pressure Measurements in Subsonic Gas Streams", NACA RM E51G09, 9/1951) and the disk-shaped static probe 130 illustrated in FIG. 1D and described by Walshe (Walshe et al., "Usefulness of Various Pressure Probes in Fluctuating Low-Speed Flow", British Aeronautical Research Council Report No. 21,714, F.M. 2917, 2/1960) were designed to approximate a portion of a wall wherein the velocity component parallel to the wall does not cause a serious problem. The lift distribution on these wing-like shapes, however, varies considerably with the cross-flow component perpendicular to the flat surfaces of the probe. On occasion it is found that pressures are even affected by the mounting sting or tube because it modifies the Kutta condition on the circular plate. For these reasons, the errors at the orifices 112 and 132 on the two flat surfaces of the probe (which are connected to axial ducts 112' and 132', respectively) do not usually compensate for one another for even small angles of incidence.
The slender cone static probe 120 illustrated in FIG. 1C and described by Huey (Huey, "A Yaw-Insensitive Static Pressure Probe", Journal of Fluids Engineering, Transaction of the ASME, Vol. 100, pp. 229-31, June 1978) has orifices 122 on the conical surface spaced at several meridian angles and connected to an axial duct 122'. This probe design assumes that flow angularity affects the pressures sensed on opposite sides of the cone in a way that provides automatic compensation. Flow across the circular cross section of the probe does, however, generate low pressures on both sides of the tube that are not compensatory.
FIG. 1E illustrates a static probe 140 having various possible cross-sectional shapes as shown by way of example in FIGS. 1F and 1G. Probe 140 has orifices 141 through its side walls connected to an axial duct 141'. The rounded-square shape 142 and the rounded-diamond shape 144, respectively illustrated in FIGS. 1F and 1G, were designed in accordance with a sophisticated design technique by Smith and Bauer (Smith et al., "Static-Pressure Probes That are Theoretically Insensitive to Pitch, Yaw and Mach Number," Journal of Fluid Mechanics, Vol. 44, Pt. 3, pp. 513-28, January 1970) and perform satisfactorily up to angles of incidence of about 10.degree..
The slotted sphere static probe 150 illustrated in FIG. 1H and described by Blackmore (Blackmore, "A Static Pressure Probe for Use in Turbulent Three-Dimensional Flows", Journal of Wind Engineering and Industrial Aerodynamics, Vol. 25, pp. 207-18, 1987) is also the result of a refined effort and is effective up to flow angles of around 20.degree.. The main element of the probe is a hollow sphere 152 having orthogonally arranged slots 154 that each subtend an angle of 110 degrees and are connected through the sphere to a duct 154'. The probe is not, however, easy to construct. Furthermore, highly turbulent flow fields require a wider range of incidence angles if the interpretation of the probe measurements is to be unrestricted and uncomplicated.
Another approach that has been used to determine static and stagnation pressures is to accept the measurement produced by a given probe and then interpret the results by means of an analysis to calculate the actual desired values. This approach would yield a true indication of the desired pressure if the proper analytical relationship could be found to account for such items as the probe shape, orifice design, stream turbulence, flow angularity, flow unsteadiness, etc., and thereby relate the measured quantities to the true pressures. Some probes yield values for the desired pressures with negligible corrections and others require that the data undergo considerable processing. The devices preferred are those that have the smallest need for correction because the technique required for analysis of the data is not always apparent. For example, it was found by Bradshaw and Goodman (Bradshaw et al., "The Effect of Turbulence on Static-Pressure Tubes," British Aeronautical Research Council Report No. 3527, September 1966) that data taken with Prandtl probes of various sizes to determine the static pressure in various jet flow fields yielded results that were greater or less than the true static pressure depending on the diameter of the probe. A relationship between the measured and actual values was not found.
Another more conventional approach is to measure the time-dependent velocities with devices such as a hot-wire probe or a Laser Velocimeter (LV), and then work out an analysis of how the pressure and velocity fluctuations are related to the measurements and to the probe design. These methods provide more information on the details of the stream than do pressure probes, but also require an extensive amount of equipment which must be carefully tuned and calibrated by experienced staff.
The research of Becker and Brown (Becker et al., "Response of Pitot Probes in Turbulent Streams", Journal of Fluid Mechanics, Vol. 62, Pt. 1, pp. 85-114, 1974) prompted the development of a new probe shape for the measurement of total head based on the streamwise component of the velocity. Their investigation accepts measurements by a probe or probes and then interprets the measurements by means of an analysis. Becker and Brown processed the total head measurements from two differently shaped probes to determine the turbulence level and the impact pressure distribution in a jet stream.
Cho and Becker (Cho et al., "Response of Static Pressure Probes in Turbulent Streams", Experiments in Fluids, Vol. 3, pp. 93-102, 1985) applied the same type of analysis as used by Becker and Brown for the determination of static pressure in a turbulent stream. In that study, the pressure measurement at a single static pressure orifice on a round probe is used to determine the time-averaged static pressure. The measurement is related by an analysis to estimate the true time-averaged static pressure. Their methods are complicated and require assumptions as to the nature of the turbulence in the flow field and as to its interaction with the probes used in the measurements. The use of a single measurement further does not provide for any redundancy in the final determination of the time-averaged static pressure and therefore lacks reliability.
U.S. Pat. No. 2,923,152 to Mabry, Jr. et al. discloses a five-prong aerodynamic pickup wherein at least three air flow variables can be detected using a single pickup. The pickup has a total pressure probe, two sideslip probes and two angle of attack probes, the probes all having the same dimensions. Signals from the pickup are routed to the cockpit of an airplane.
U.S. Pat. No. 4,184,149 to Baker et al. discloses a single air speed and attitude probe wherein only total pressure and static pressure are determined. The probe has a tubular body. A static pressure transducer and a total pressure transducer are supported within the probe body.
U.S. Pat. No. 4,833,917 to Wilson discloses a three component velocity probe for large scale applications wherein three sensing holes of the same dimension are located on one probe. The arrangement allows a three dimensional representation of a velocity field to be calculated. The probe detects static pressure as well as total pressure.
While several attempts have been made to design probes that are insensitive to the angle of incidence and to turbulence fluctuations, including the design of specially shaped probes, the probes are difficult to build. Thus, a very limited amount of testing of the designs has been carried out. The unusual probe shapes make data interpretation complicated. The probes have had limited success at larger angles of incidence, and thus do not perform well in a turbulent stream.